Cos'? Una Cella a Combustibile?


The easiest way to understand fuel cells is to think of them as a cousin to the ordinary battery. Both produce electricity through electrochemical reactions. The difference lies in a fuel cell's ability to constantly produce electricity as long as it has a source of fuel where a battery needs to be recharged. Consequently, since a fuel cell does not store energy internally, a fuel cell will not "run down" like a battery. Fuel cells directly convert the fuel into electricity where a battery has to replenish its electricity from an external source.

The fuels utilized by a fuel cell to generate electricity are hydrogen and oxygen. Hydrogen, the most abundant element on Earth, is rarely found in its pure form. Most fuel cell systems employ a component called a reformer to extract hydrogen from hydrogen rich fossil fuels. The by-products of this process are carbon dioxide, less than half the amount generated by traditional electricity generation methods, and trace amounts of nitrous oxide. The hydrogen purity requirement and the need for reforming are dependent on the type of fuel cell stack employed.

In order to provide an example of the electrochemical process that occurs in a fuel cell, the following describes the chemical reactions in a typical proton exchange membrane (PEM) fuel cell. Once the fuel has been reformed into hydrogen, the fuel cell combines oxygen, from the surrounding atmosphere, and hydrogen to generate electricity and water. The hydrogen is fed into the anode side of the cell where it encounters a catalyst. The catalyst strips the negatively charged electrons from the hydrogen, which are then routed out of the cell through an external circuit (i.e. light bulb, house, motor, etc?). The hydrogen ions (H+) travel through the electrolyte contained in the fuel cell until they reach the cathode. Once at the cathode, the hydrogen ion (H+), the electron that traveled through the external circuit and the oxygen molecule join together. The by-products of the electrochemical reaction that occurs in a fuel cell are electricity, water vapor and heat. Theoretically, the water vapor can be recycled to produce additional hydrogen. The waste heat can be utilized for heating water, space heating and cooling. The direct conversion of fuel into electricity allows fuel cells to achieve substantially higher efficiencies than combustion, which is limited by Carnot's Law of Thermodynamics. Fuel cells achieve efficiencies of 35% to 90% depending on whether the waste heat is employed. These efficiencies are about 2 to 3 times higher than a combustion engine which converts fuel to heat, then into mechanical energy and finally into electricity.

The final major component of a fuel cell system is the power conditioning equipment. This piece converts the low-voltage DC power produced by a fuel cell into high voltage AC power, which most household appliances operate on. Many fuel cell power conditioning units employ batteries to handle peak demand loads that are beyond the fuel cell system's peak output. This can occur when multiple appliances are started at once. The power conditioning unit also controls the electricity's frequency and maintains the harmonics to an acceptable level.